This invention pertains to the art of methods and apparatuses for use with the extrusion of visco-elastic materials, and more specifically to the art of methods and apparatuses for the control and reduction of extrudate shrinkage.
It is well known in the art to extrude visco-elastic materials such as rubber, plastics and food products using one or more extruders individually or in combination. Visco-elastic materials possess both viscous (liquid-like) and elastic (solid-like) characteristics. During processing, visco-elastic materials flow (like liquids), but they also experience stretching (like elastic solids). This stretching of the visco-elastic material while it flows is called elongational flow. A flow channel is provided for communicating the visco-elastic material from the extruder to a die. Typically, the flow channel is provided within an extruder head. It is also common to add additional flow channels in die assemblies which may have splice bars, inserts, preformers and the like to communicate the visco-elastic material from the extruder head to the die. A die plate provides the final cross-sectional contour of the extrudate. When a roller is used with a die assembly to form the final cross-sectional shape, the extrusion apparatus is referred to as a roller die extrusion apparatus. Examples of roller die extrusion apparatus are shown and described in U.S. Pat. Nos. 4,526,528 and 4,539,169. Visco-elastic materials undergoing the extrusion process are referred to as xe2x80x9cmeltxe2x80x9d while the same materials after the extrusion process are referred to as xe2x80x9cextrudatexe2x80x9d.
Conventional methodology, known in the rubber industry, uses flow channels with continually decreasing cross sectional areas, i.e., xe2x80x9cfunnelxe2x80x9d like cross sections. This is done to maintain the melt in compression and to prevent xe2x80x9cdead spotsxe2x80x9d and any resulting porosity in the extrudate. Typically in the tire industry, the flow channels in the extruder head are designed to the maximum width (or sum of the tire components widths) and to the maximum thickness of the tire components to be extruded. This results in die assembly flow channels that begin with a very wide, thick cross sectional area and end with a very small, thin cross sectional area. All such xe2x80x9cfunnel-likexe2x80x9d flow channels lead to increased elongational flow as will be discussed further below.
One problem known in the art, for example in the rubber industry, is referred to as die swell. Die swell is the difference between the dimensions of the cross section of an extrudate and the corresponding dimensions of the die orifice by which the extrudate is formed. This definition is found in ASTM D1566-93a, xe2x80x9cStandard Terminology Relating to Rubber.xe2x80x9d Typically, die swell is undesirable as it leads to extrudate that has different dimensions from the xe2x80x9cas extrudedxe2x80x9d or xe2x80x9con specxe2x80x9d dimensions.
Another problem know in the art is shrinkage. Shrinkage is the tendency of the extrudate to become shorter in the process direction as a function of time and temperature. The more the melt is stretched during the extrusions process, i.e., the greater the elongational flow, the more the extrudate will tend to shrink after exiting the die. This tendency of visco-elastic material to revert after processing to the state prior to processing is referred to as xe2x80x9cmemory.xe2x80x9d It is as if after the extrusion process the visco-elastic material xe2x80x9cremembersxe2x80x9d its molecular state before the extrusion process and tries to return to it. Thus, the extrudate shrinks. Shrinkage is undesirable because it leads to extrudate that is shorter and/or of less thickness than the xe2x80x9cas extrudedxe2x80x9d or xe2x80x9con specxe2x80x9d dimensions. In the tire industry, shrinkage can result in open component splices on green tire constructions and in cured tires. This occurs because the tire component shrinks, i.e., gets shorter in overall length, leading to a tire component that does not go all the way around the tire so that a proper splice can be made or maintained.
Still another problem with shrinkage, known in the tire industry, occurs when tire components are formed from different rubber compounds which have been co-extruded. Each compound is initially processed through a separate extruder but then all the compounds, as melts, are brought together, typically using a common extruder head or die assembly of some kind, so that they can be extruded together with each compound properly placed with respect to the other compounds. The problem is related to the fact that the amount of shrinkage of each tire compound depends on the rubber composition and the size of the extruder used to make it. In other words, each rubber compound has a unique shrinkage and rate of shrinkage (depending on the extruder used and die assembly). For example, if a tire component made of a higher shrinkage rubber compound, e.g., white sidewall, is co-extruded with a tire component made of a lower shrinkage rubber compound, e.g., black sidewall, then the resulting extrudate, after being cut and given sufficient time, may buckle or curl (depending on the geometry of the components). This would occur because the white sidewall compound would shrink more than the black sidewall compound. The resulting tire would be a reject because during the curing of the tire, the black sidewall compound would fill any gap created by the higher shrinkage white sidewall compound producing a tire with a discontinuous white sidewall.
The present invention provides methods and apparatuses for reducing and/or controlling the problems mentioned above. Thus, the difficulties inherent in the prior art are overcome in a way that is simple and efficient while providing better and more advantageous results.
In accordance with one aspect of the invention there is provided an extruder head having a head flow channel for communicating and shaping a visco-elastic melt. The head flow channel has a low elongational flow zone at least 1.0 inch long at the end of the head flow channel.
In accordance with another aspect of the invention, the low elongational flow zone has a varying cross sectional shape and a length ratio within the range of 0.10 to 0.50. The length ratio is the ratio of the length of the low elongational flow zone to the length of the head flow channel.
In accordance with still another aspect of the invention there is provided a die assembly having a flow channel for communicating and shaping a visco-elastic melt. The flow channel has a low elongational flow zone at least 1.0 inch long.
In accordance with another aspect of the invention, the low elongational flow zone has a length divided by area (L/A) ratio of at least 2.0. The length divided by area (L/A) ratio is the ratio of the length of the low elongational flow zone to the cross sectional area of the low elongational flow zone.
In accordance with another aspect of the invention, the flow channel of the die assembly has a flow channel area ratio of the cross sectional area of the flow channel at its entrance to the cross sectional area of the flow channel at its exit from the die within the range of 1.0 to 3.0.
In accordance with still another aspect of the invention there is provided a method of reducing the shrinkage of a visco-elastic extrudate formed from a visco-elastic melt wherein the visco-elastic melt flows through an extruder head having a head flow channel with a low elongational flow zone and then flows through a die assembly having a first flow channel with a low elongational flow zone.
In accordance with another aspect of the invention there is provided a method of controlling the die swell of a visco-elastic melt wherein the visco-elastic melt flows through a flow channel in a die assembly at a melt speed V1. The visco-elastic melt then flows onto a roller rotating at a roller speed V2 which is less than the melt speed V1. The melt speed V1 and the roller speed V2 define a speed ratio of V1/V2 which is at least 1.1.
One advantage of the present invention is that elongation flow of a visco-elastic melt is reduced thereby reducing the shrinkage of the resulting extrudate.
Another advantage of the present invention is that the shrinkage differential between different rubber compounds is reduced.
Still another advantage of the present invention is that die swell is controlled and thereby shrinkage of the extrudate is reduced.
Still other benefits and advantages of the invention will become apparent to those skilled in the art to which it pertains upon a reading and understanding of the following detailed specification.